Industrial coatings are surface protective coatings (paint coatings) applied to substrates and typically cured or crosslinked to form continuous films for decorative and protective purposes. A protective coating ordinarily comprises an organic polymeric binder, pigments, and various paint additives, where the polymeric binder acts as a fluid vehicle for the pigments and imparts fluid rheological properties to the liquid paint coating. Upon curing or crosslinking, the polymeric binder hardens and functions as a solid binder for the pigments and provides adhesion of the dried paint film to the substrate. The pigments may be organic or inorganic and functionally contribute to opacity and color in addition to providing durability and hardness. Protective coatings which contain little or no opacifying pigments are described as clear coatings. The manufacture of protective coatings involves the preparation of a polymeric binder, mixing of component materials, grinding of pigments in the polymeric binder, and thinning to commercial standards to provide a fluid coating.
Water dispersed coatings compositions containing epoxy resin are frequently used for coating cans and containers including plastic bottles. A typical plastic bottle is manufactured from polyethylene terepethalate (PET) thermoplastic molding resins. Although PET bottles and similar plastics are quite commonly used, thermoplastics provide a serious disposal problem since plastics do not disintegrate in landfills. Consequently PET and similar plastic containers are recycled either through a shredding process and subsequently reused as a low level plastic or subjected to a caustic cleaning where plastic containers are cleaned and reused. In either process, the originally applied coating must be removed, preferably with caustic treatment. Typical prior art coatings used for plastic bottles and containers often comprise polyamine crosslinked polyepoxide thermosetting resins, which frequently are permanently adhered to the plastic and cannot be removed by a caustic cleaning processes.
It now has been found that strippable acrylic water borne ionomeric latex coatings are particularly useful for coating PET and other plastic bottles, as well as plastic substrates generally, especially when the plastic is recycled. The cured ionomeric coatings are unexpectedly flexible, tough, very high in gloss, smooth, moisture resistant, heat pasteurization resistant, and resistant to boiling water. The ionomeric coatings are compatible with processes used to coat and recycle PET and similar plastics. The ionomeric coatings of this invention can be tinted to provide color to the plastic bottle, but leave the PET uncolored, and thus provide a greater value for recycling. The ionomeric coatings can provide a suitable barrier to oxygen ingress and carbon dioxide loss if a high ratio of pigments is added to enhance barrier properties. Caustic used to clean PET bottles prior to recycling will easily remove these strippable coatings. After recycling, new ionomeric coatings can be applied to plastic bottles to give a new fresh surface. The strippable ionomeric coating protects the PET from scratches and abrasion and thus prolonging the useful life of the PET bottle.
The aqueous strippable ionomeric coatings of this invention are based on a zinc or zirconium ammonium complexed ionomeric polymeric latex binder comprising a carboxylic acid functional polymer neutralized with ammonium and blended with a zinc or zirconium ammonium complex. An ionomer can be defined as a polymer composed of a polymeric backbone containing pendant carboxylic acid groups, which are ionically bound partially or completely with zinc or zirconium metal cations to form an ionomer. Aqueous coatings based on blends of amine neutralized carboxyl polymer and metal ammonia complex were found to be particularly stable in an aqueous mixture provided both components in the blend are fully neutralized. The neutralized blend of carboxyl polymer and zinc ammonia carbonate for instance do not interact until after the water and ammonia volatilize in the heat curing cycle, where the carboxyl functional polymer forms ionic crosslinks with the remaining zinc or zirconium cations. The ionomeric polymer was found to provide interreacting polymer chains which exhibit thermosetting properties comparable to coreactive polymeric binders crosslined by a crosslinker. These ionic moities and their interactions dominate the behavior of the polymer itself where it is believed that the cation is exchanged for a hydrogen ion of the polymer carboxyl group. Ionic hydrocarbon polymers for elastomers or plastics are disclosed in U.S. Pat. No. 3,264,272. Non-aqueous liquid and powder coatings useful as thermoset coatings generally are shown in commonly assigned U.S. Pat. No. 5,025,063, U.S. Pat. No. 5,306,134, U.S. Pat. No. 5,155,162, U.S. Pat. No. 5,157,073, U.S. Pat. No. 5,034,452, and U.S. Pat. No. 5,762,427.
In accordance with this invention, improvements in the properties of polymeric binders for paint coatings can be achieved through the interaction of the metal cation ammonia complex of zinc or zirconium reacted with ammonia neutralized carboxyl functional, aqueous emulsion polymers. The modification of polymer properties is believed due to aggregation of ions described as ionic crosslinking, where the coreaction of two ion pairs on adjacent polymer chains results in a four-centered aggregate and behaves essentially like a crosslink. The formation of ionomeric clusters or multiplets in protective surface coatings causes the carboxylic acid functional polymers to behave like a phase-separated block copolymers.
Paint coating compositions based on the ionomeric polymers provide an increase in the moduli, an increase in glass transition temperatures, and an increase in hardness. The actual cluster or multiplet formation takes place during the bake cycle at temperatures above the temperature where the zinc or zirconium ammonium complex salt will lose its acidified counterion. Thus, the addition of low levels of metal ammonia complex to carboxyl acid functional emulsion polymeric binders has been found to considerably improve on the physical properties of the carboxyl polymer. Incorporation of zinc ammonia carbonate, for instance, causes a thermoplastic acrylic ionomer to exhibit cured film properties such as solvent resistance and hardness comparable to a conventionally cured crosslinked paint film. Useful carboxyl polymers characteristically exhibit low polarity, high hydrophobicity, and low hydrogen bonding characteristics. Zinc and zirconium ammonium carbonate complexes are preferred and generally resist water or humidity and produce water resistant coatings. The acrylic ionomeric coatings are aqueous based, removable with caustic, contain zero VOC, and provide negligible toxicity other than ammonia. These and other advantages of the invention will become more apparent by referring the detailed description of the invention and illustrative examples.